Regarding scanning the incoming objects for EM emissions...
So lets say this object is heading straight for us. Who would they need to contact? Most likely the beings back at the destination. Why would one waste the energy to broadcast EM out the entire universe when they could simply send something like a pulsed laser backwards directly to their origin point. Likewise, if they wanted to talk to us, some sort of directed beam would make it quite clear we were the intended recipients.
For one thing, it saves energy. For another, if there really are other races out there, maybe they aren’t all friendly so advertising your position by broadcasting isn’t prudent.
Lastly, what exactly would they be saying? They’re travelling at sub light speeds. Those at the destination, if still alive, can see them and whether they’ve arrived. Barring periodic status updates, what would they be chatting about for us to intercept?
Unless they’re cosmic drug dealers with associated cosmic boom boxes to advertise their wares.
The scouts (Oumuamua and its companion) passed through the system and confirmed that there is competition located on the 3rd planet.
Now the casual seed (C/2019) comes this way, bringing absolute planetary or even system-wide destruction, and the elimination of one more competitor.
S Hawking was right - the Dark Forest has come calling, and here is no place to hide.